C3 and C4 olefins, such as propylene (propene), 1-butene, isobutene and butadiene, are widely used starting materials in the industrial synthesis of a variety of important chemical products. The principal industrial method for producing C3 and C4 olefins is steam cracking, a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. The products obtained by steam cracking depend on the composition of the feed, the hydrocarbon-to-steam ratio, and on the cracking temperature and furnace residence time. For example, a feed composition that primarily contains ethane (ethane cracking) would result in high ethylene yields, while a feed composition including larger hydrocarbons, such as naptha (naptha cracking), would result in a larger yield of C3 and C4 olefins.
Over the last decade, the demand for C3 and C4 olefins has outstripped supply from traditional cracker units, and this trend is expected to accelerate over the next decade. For example, the current world demand for propene is around 100 million metric tons per year (MTA), and is expected to increase significantly over the next five years. This trend is primarily due to the availability of cheap shale gas, prompting many chemical companies to convert their naphtha crackers into ethane crackers, thus shifting production towards ethylene and away from longer chain C3 and C4 olefins. Accordingly, the demand for C3 and C4 olefins is growing faster than can be supplied by only cracking.
Because C3 and C4 olefin production by conventional steam cracking has not kept pace with rising demand, several alternative “on-purpose” olefin production technologies that convert short chain alkanes to the corresponding olefins have been developed. Examples include the catalytic dehydrogenation (DH) of short chain alkanes, such as propane, to the corresponding olefin, such as propene, using a supported CrOx/Al2O3 catalyst (“CATOFIN®” (Lummus)), a Pt/Sn alloy supported on Al2O3 (“OLEFLEX™” (UOP)), or Pt/Sn supported on Zn-aluminate with co-fed steam (“STAR®” (Uhde)) (see Sattler et al., Chem. Rev., 2014, 114 (20), 10613-10653).
These and other currently used on-purpose dehydrogenation technologies are energy intensive, because the dehydrogenation reaction is highly endothermic. Furthermore, because they require high temperature conditions, they result in substantial catalyst deactivation due to the formation of coke. Thus, they require continuous catalyst regeneration. In addition, these processes may require substantially reduced pressure to shift the dehydrogenation equilibrium towards the desired products, further contributing to the high production costs associated with these methods.
Oxidative dehydrogenation (ODH), the catalytic dehydrogenation of feedstock alkanes in the presence of oxygen, is an intriguing alternative to conventional dehydrogenation that addresses each of the disadvantages of current DH technology. When oxygen is co-fed to act as a reactant, the reaction thermodynamics are altered such that the resulting net reaction is exothermic. Accordingly, the reaction can proceed at much lower reaction temperatures, resulting in decreased energy costs and increased catalyst stability. Oxygen in the feed stream also eliminates coke formation on the catalyst surface and thus creates no need for catalyst regeneration.
Despite these purported advantages, industrial-scale ODH processes have not been implemented, due to poor control of unwanted side-reactions (mainly the over-oxidation of olefin to CO and CO2), which results in low olefin selectivity at conversions necessary for industrial implementation. For example, existing catalysts for propane ODH typically provide ˜50-60% selectivity to propene at 10% propane conversion, with the byproducts largely made up of CO and CO2. As a result, even after more than 30 years of research into catalysis development for ODH (almost entirely focused on supporting vanadium oxide on amorphous oxide supports (e.g., SiO2, Al2O3, TiO2, CeO2, ZrO2) and structured oxides (e.g., MCM-41, SBA-15)), ODH has not been successfully used in the industrial-scale production of C3 and C4 olefins.
Accordingly, there is a need in the art for improved methods and catalysts for the oxidative dehydrogenation of C3-C5 alkanes to the corresponding olefins.